The polymerization process of an olefin is generally classified into a high pressure process, a solution process, a slurry process, a gas-phase process, and so on, and there have been various efforts to prepare olefin polymers having desired properties by applying various metallocene catalysts to the polymerization process.
The metallocene catalyst for the preparation method of polyethylene using a slurry and gas-phase polymerization process must be firmly fixed to a suitable carrier in order to not generate fouling due to leaching in a reactor. Particularly, the bulk density of the polymer relates to productivity per reactor, and thus not only the activity of the catalyst but also the bulk density of the polymer must be high.
It is usual in the preparation of a supported metallocene catalyst not only to use a highly active metallocene catalyst for increasing the catalytic activity but also to increase the amount of aluminoxane, a cocatalyst, supported on a carrier. However, in the case of a highly active supported catalyst, generally, a hollow polymer is formed and the bulk density of the prepared polymer decreases because the polymerization occurs first at the surface of the carrier and the formed polymer is crystallized and hinders the monomers from diffusion. For resolving this problem, there has been an attempt for controlling the diffusion speed of the monomer such as ethylene into the carrier by carrying out pre-polymerization first at a low temperature and a low pressure, but there was a problem of installing an additional polymerization reactor. Further, there was a method of treating hydroxy groups of the surface of the carrier with aluminum chloride and so on for increasing the efficiency of loading, but it increases the catalyst preparation cost and may decrease the uniformity of the catalyst due to a side-reaction.